Piezoelectric ceramic transformer

ABSTRACT

One driver having external electrodes arranged over both surfaces of a piezoelectric ceramic plate polarized in the direction of its thickness, and one generator having two mutually opposed interdigital electrodes on one or both surfaces of a piezoelectric ceramic plate and alternately polarized in the direction of its length between the electrode fingers of the interdigital electrodes are formed combined in alignment in the direction of the length of the piezoelectric ceramic plate. Alternatively, the driver and generator may be formed in alignment in the direction of the width of piezoelectric ceramic plate. By applying a low voltage to this piezoelectric ceramic transformer from the driver at a resonance frequency of a longitudinal vibration of second order mode of the piezoelectric ceramic plate, a high voltage can be picked up from the generator. Here, through connection of the input terminals and output terminals at the nodes of vibration, failure due to vibration of lead wires can be prevented, thereby obtaining a compact and slim transformer element that provides high reliability. A plurality of drivers and generators may also be employed, and in addition, an interdigital electrode may be used in the driver as well as in the generator.

This is a divisional of application Ser. No. 08/348,925 filed Nov. 25,1994 U.S. Pat. No. 5,576,590.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric ceramic transformerused in various types of power circuits for generating high voltage, andparticularly to a highly reliable piezoelectric ceramic transformerwhich is thin and compact, and which also generates high voltage.

2. Description of the Related Art

In recent years, wound-type electromagnetic transformers have been usedfor generating high voltage in internal power circuits of devices suchas television deflecting devices or charging devices of copiers whichrequire for high voltage. Such electromagnetic transformers take theform of a conductor wound onto a core of magnetic substance. Because alarge number of turns of the conductor are required to realize a hightransformation ratio, electromagnetic transformers that are compact andslim in shape are extremely difficult to produce.

To remedy this problem, piezoelectric transformers utilizing thepiezoelectric effect have been provided. FIG. 1 shows the constructionof a Rosen-type piezoelectric transformer, a representative example of apiezoelectric transformer of the prior art. For producing high voltage,the portion indicated 41 in this piezoelectric ceramic transformer isits low-impedance driver provided with electrodes 43, 44 on its upperand lower surfaces, this portion being polarized in the direction of thethickness of a piezoelectric plate as shown by the arrow 49 in thefigure. The portion indicated 42 in the figure is a high-impedancegenerator provided with electrode 45 at its end, the generator 42 beingpolarized along the length of the piezoelectric plate as shown by thearrow 50 in the figure. This piezoelectric transformer operates asfollows: When voltage is impressed to drive electrodes 43, 44 fromexternal terminals 46, 47, an electric field increases in the directionof polarization, and a longitudinal vibration in the longitudinaldirection is excited by the piezoelectric effect displaced in adirection perpendicular to polarization (hereinafter abbreviated"piezoelectric transverse effect 31 mode"), whereby the entiretransformer vibrates. Moreover, in generator 42, due to thepiezoelectric effect generating a potential difference in thepolarization direction (hereinafter abbreviated "piezoelectriclongitudinal effect 33 mode") which is caused by a mechanical stainoccurring in the polarization direction, a voltage is produced which hasthe same frequency as the input voltage from output electrode 45 toexternal terminal 48. At this time, if the drive frequency is made equalto the resonance frequency of the piezoelectric transformer, anextremely high output voltage can be obtained. Furthermore, forinputting high voltage and outputting low voltage, the high-impedancesection 42 of longitudinal effect can obviously be made the input sideand the low-impedance section 41 of transverse effect be made the outputside.

This piezoelectric transformer is used in a resonant state, and comparedwith ordinary electromagnetic transformers, has numerous advantagesincluding: 1) a compact and slim shape can be achieved because awound-type construction is not required and energy density is high; 2)non-combustibility is possible; and 3) there is no electromagneticinduction noise. Nevertheless, in this Rosen-type piezoelectrictransformer of the prior art, the electrode of the generator portion islocated at the end of the transformer, i.e., at the loop of vibration,and external electric terminal of lead wire must also be led out fromthis portion. In such a case, because the mass of the terminal of thelead wire as well as connection component such as solder lies at theloop of vibration, there is an increase in mechanical loss and huntingof frequency characteristics during resonance. Increase in mechanicalloss causes a drop in efficiency, while hunting in frequencycharacteristic causes instability in circuit operation, both problemsposing serious obstacles to putting such a transformer into practicaluse.

In addition, in contrast with a piezoelectric element used for signalprocessing such as in filters, a piezoelectric transformer must operateat relatively high power and is caused to vibrate at large amplitudesapproaching the capacity limits of piezoelectric materials. In such apiezoelectric transformer, the location of the connection components atthe loop of vibration means that the connection components receive thebrunt of large vibrations, with the result that, despite the use ofconnection methods such as soldering or bonding, the reliability of theconnection components is severely compromised in terms of lifeexpectancy.

There is the further disadvantage that, although a relatively highoutput voltage can be obtained when the load resistance value issignificantly greater than the piezoelectric transformer impedance, whenthe load resistance value is not so large, a particularly high outputvoltage cannot be obtained.

Furthermore, as is clear from FIG. 1, there is the problem that thisRosen-type piezoelectric transformer is of three-terminal constructionand electrical insulation between the input and output cannot beachieved.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a piezoelectricceramic transformer that solves the above-described problems, that hasthe characteristics of high voltage, high power, and high reliability,and which furthermore is slim and compact.

To achieve this object, the first invention of the piezoelectric ceramictransformer comprises a piezoelectric ceramic plate which is apiezoelectric vibration material in the form of a rectangular platecomprising at least one driver and at least one generator, each of thedrivers and generators being arranged in a line in arbitrary order alongthe longitudinal direction; input terminals for applying a commonvoltage to each driver; and output terminals for picking up a commonvoltage from each generator; each driver having a pair of opposingexternal electrodes, one on each side of the piezoelectric ceramic platein the direction of thickness, the piezoelectric ceramic plate of thedriver being polarized in the direction of thickness, each generatorhaving an interdigital electrode assembly provided on at least onesurface of the piezoelectric ceramic plate in the direction ofthickness; one part of the interdigital electrode assembly being made upof a plurality of electrode fingers spaced at intervals and parallel tothe direction of width of the piezoelectric ceramic plate, and aconnecting electrode linking these electrode fingers; this part andanother part of similar construction being arranged such that electrodefingers of both parts alternately project so as to oppose each otheracross the width of the piezoelectric ceramic plate; and moreover, thepiezoelectric ceramic plate of the generator being polarized inalternating longitudinal directions between each adjacent pair ofelectrode fingers; the input terminals being connected to the pair ofexternal electrodes of each driver and the output terminals beingconnected to the interdigital electrode assembly of each generator, allinput and output terminals being connected at points that are nodes ofvibration when the piezoelectric ceramic plate is driven at longitudinalvibration resonance frequency of the piezoelectric ceramic plate.

The second invention of the piezoelectric ceramic transformer comprisesa piezoelectric ceramic plate which is a piezoelectric vibrationmaterial in the form of a rectangular plate, in the direction of widthof which are arranged one driver and one generator; input terminals forapplying a voltage to the driver; and output terminals for picking upvoltage from the generator; the driver having a pair of opposingexternal electrodes, one on each side of the piezoelectric ceramic platein the direction of thickness, and the piezoelectric ceramic plate ofthe driver being polarized in the direction of thickness, the generatorhaving an interdigital electrode assembly provided on at least onesurface of the piezoelectric ceramic plate in the direction ofthickness; one part of the interdigital electrode assembly being made upof a plurality of electrode fingers spaced at intervals and parallel tothe direction of width of the piezoelectric ceramic plate, and aconnecting electrode linking these electrode fingers; this part andanother part of similar construction being arranged such that electrodefingers of both parts alternately project so as to oppose each otheracross the width of the piezoelectric ceramic plate; and moreover, thepiezoelectric ceramic plate of the generator being polarized inalternating longitudinal directions between each adjacent pair ofelectrode fingers; the input terminals and the output terminals beingconnected to the external electrode pair and the interdigital electrodeassembly, respectively, at the center of the driver and generator,respectively, in the direction of length of the piezoelectric ceramicplate.

The third invention of the piezoelectric ceramic transformer comprises apiezoelectric ceramic plate which is a piezoelectric vibration materialin the form of a rectangular plate comprising at least one driver and atleast one generator, each of the drivers and generators being arrangedin a line in arbitrary order along the longitudinal direction; inputterminals for applying a common voltage to each driver; and outputterminals for picking up a common voltage from each generator; each ofthe drivers and the generators having an interdigital electrode assemblyprovided on at least one surface of the piezoelectric ceramic plate; onepart of each interdigital electrode assembly being made up of aplurality of electrode fingers spaced at intervals and parallel to thedirection of width of the piezoelectric ceramic plate, and a connectingelectrode linking these electrode fingers; this part and another part ofsimilar construction being arranged such that electrode fingers of bothparts alternately project so as to oppose each other across the width ofthe piezoelectric ceramic plate; and moreover, the piezoelectric ceramicplate of each of the drivers and the generators being polarized inalternating longitudinal directions between each adjacent pair ofelectrode fingers; the input terminals and the output terminals beingconnected to the corresponding interdigital electrode assembly at thecenter of each of the drivers and the generators, respectively, in thedirection of length of the piezoelectric ceramic plate.

The fourth invention of the piezoelectric ceramic transformer comprisesa piezoelectric ceramic plate which is a piezoelectric vibrationmaterial in the form of a rectangular plate, in the direction of lengthof which are arranged one driver and one generator; input terminals forapplying a voltage to the driver; and output terminals for picking upvoltage from the generator; the driver having an interdigital electrodeassembly provided on one surface of the piezoelectric ceramic plate; onepart of the interdigital electrode assembly being made up of a pluralityof electrode fingers spaced at intervals and parallel to the directionof width of the piezoelectric ceramic plate and a connecting electrodelinking these electrode fingers; this part and another part of similarconstruction being arranged such that electrode fingers of both partsalternately project so as to oppose each other across the width of thepiezoelectric ceramic plate; and moreover, the piezoelectric ceramicplate of the driver being polarized in alternating longitudinaldirections between each adjacent pair of electrode fingers; and thegenerator being made up of one strip electrode arranged at a position1/4 of the length of the entire piezoelectric ceramic plate from one endsurface of the piezoelectric ceramic plate opposite the driver side inthe longitudinal direction, the piezoelectric ceramic plate of thegenerator being polarized in the longitudinal direction between thedriver and this strip electrode; and further, the input terminals beingconnected to the interdigital electrode assembly at the center of thepiezoelectric ceramic plate of the driver in the direction of length,and one of the output terminals being connected to one of the ends ofthe strip electrode and to one of the input terminals and the otheroutput terminal being led out from the other end of the strip electrode.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description based onthe accompanying drawings which illustrate examples of preferredembodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the construction of a Rosen-typepiezoelectric transformer of the prior art;

FIG. 2(a) is a plan view showing the construction of a first embodimentof the piezoelectric ceramic transformer according to the presentinvention, and FIG. 2(b) is a sectional view of the same embodiment;

FIG. 3(a) is a connection diagram of the external terminals of thesecond embodiment of the piezoelectric ceramic transformer according tothe present invention, and FIG. 3(b) shows the displacementdistribution;

FIG. 4 is a circuit diagram of the lumped constant approximateequivalent circuit of the piezoelectric ceramic transformer according tothe present invention

FIG. 5 is a perspective view showing the construction of the thirdembodiment of the piezoelectric ceramic transformer according to thepresent invention;

FIG. 6 is a plan view of the fourth embodiment of the piezoelectricceramic transformer according to the present invention;

FIG. 7 is a plan view of the fifth embodiment of the piezoelectricceramic transformer according to the present invention;

FIG. 8 shows an example of manufacturing method of interdigitalelectrodes for a piezoelectric ceramic transformer;

FIG. 9(a) shows the first step of a manufacturing method forinterdigital electrodes of each of the embodiments, and FIG. 9(b) showsthe second step of the manufacturing method for interdigital electrodesof each of the embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A plan view of the first embodiment of a piezoelectric ceramictransformer according to the present invention is shown in FIG. 2(a) anda sectional view is shown in FIG. 2(b). Driver 11 is made up ofpiezoelectric ceramic plate 17 polarized in the direction of thicknessas shown by the arrow in FIG. 2(b), and one external electrode 131, 132arranged on each of the upper and lower main surfaces of piezoelectricceramic plate 17; and external electrical terminals 151, 152 leadingaway from the central portion in the longitudinal direction of externalelectrodes 131, 132, respectively. In the generator 12, an assembly ofopposing interdigital electrodes 141, 142 are arranged on each of theupper and lower main surfaces of piezoelectric ceramic plate 18. Betweeninterdigital electrode fingers 141a, 142a of interdigital electrodes141, 142, piezoelectric ceramic plate 18 is polarized in thelongitudinal direction in alternately switching polarities as shown bythe arrows in the figure; and on both upper and lower main surfaces,external electrical terminals 161, 162 lead from the electrode finger141a arranged at the central portion of interdigital electrode 141 andfrom the central portion in the longitudinal direction of interdigitalelectrode 142, respectively.

Explanation regarding the lead-out location of the external terminalsand the driving frequency of the present piezoelectric ceramictransformer will be presented together with that of the secondembodiment. Here, the operation of the present embodiment will beexplained.

When an alternating voltage of a frequency in the vicinity of theresonance frequency of a longitudinal vibration is applied betweenexternal terminals 151-152 of driver 11 of the construction shown inFIG. 2, longitudinal vibration in the longitudinal direction isgenerated by the piezoelectric transverse effect 31 mode through theelectromechanical coupling factor k₃₁. This longitudinal vibration ispropagated in generator 12, a voltage is generated between interdigitalelectrodes 141-142 by piezoelectric longitudinal effect 33 mode due toelectromechanical coupling factor k₃₃, and this voltage is led out fromexternal terminals 161-162. At this time, since the frequency of theapplied voltage is equal to the resonance frequency of longitudinalvibration of the piezoelectric ceramic transformer, a rather high outputvoltage can be obtained. Further, as shown in FIG. 2(b), interdigitalelectrodes are arranged in matching positions on both the upper andlower main surfaces, but operation is still possible at a slightdecrease in generation effect even if the interdigital electrodes arearranged on only one main surface.

A second embodiment of the present invention is shown in FIG. 3. In thepresent embodiment, two each of the driver 11 and generator 12 describedin the first embodiment are formed in a line in an alternatingarrangement on the piezoelectric ceramic plate in the direction oflength. FIG. 3(a) shows the connection points of the external terminalsof this piezoelectric ceramic transformer. FIG. 3(b) shows thedisplacement distribution of the longitudinal vibration in thelongitudinal direction of the (2+2), i.e., fourth order in thelongitudinal direction of this piezoelectric ceramic transformer. As isclear from the figure, the lead-out points of external terminals 151,152, 153, 154, 161, 162, 163, and 164 coincide with vibration nodes A,B, C, and D.

Generally, in longitudinal vibration of pth order, vibration nodes occurat the centers of a number p of divisions into which the length of thepiezoelectric ceramic transformer is divided. Accordingly, in apiezoelectric ceramic transformer made up of m drivers and n generators,if driven at the resonance frequency of longitudinal vibration of(m+n)th order in the longitudinal direction, all of the externalterminals can be led out from the vibration nodes, thereby achievingexcellent vibration characteristics and high reliability. Here, althoughtwo drivers and two generators are alternately connected as an example,the present piezoelectric ceramic transformer will operate if there areone or more of each of the driver and generator, and the drivers andgenerators need not be connected alternately. Moreover, the electricalconnections of each of the plurality of drivers and generators may beserial, parallel, or a combination, as long as the connections do notnullify electrical charge.

The lumped constant approximate equivalent circuit of the resonantfrequency vicinity of the piezoelectric ceramic transformer of thepresent embodiment, as well as of the first embodiment, is shown in FIG.4. In FIG. 4, Cd₁ and Cd₂ are the damped capacities of the input sideand output side, respectively; A₁ and A₂ are the input force factor andoutput force factor, respectively; and m, c, and r_(m) are theequivalent mass, equivalent compliance, and equivalent mechanicalresistance, respectively, of the concerned longitudinal vibration mode.The input and output force factors A1, A2 of the piezoelectric ceramictransformer of the present invention change with the width and thicknessof the transformer, distance between the electrode fingers, and numberof the electrode fingers. As is clear from the equivalent circuit ofFIG. 4, output voltage V_(out) of the piezoelectric ceramic transformergenerally changes according to the resistance of a connected load, andthe greater the load resistance value, the greater the value of V_(out).In addition, the energy transmission efficiency depends on the loadresistance, and at loads other than a value matching the outputimpedance of the piezoelectric ceramic transformer, transmissionefficiency is not very high. In a piezoelectric ceramic transformeraccording to the present invention, there is a degree of freedomregarding not only the overall length (the number of drivers andgenerators), width, and thickness of the transformer, but also thenumber of electrode fingers or the distance between the electrodefingers, and such a transformer therefore is characterized by a widerange over which the load and the output impedance of the piezoelectricceramic transformer can be matched.

In addition, as is clear from FIG. 2 and FIG. 4, the presentpiezoelectric ceramic transformer forms a 4-terminal structure in whichthe external terminals for input and output are electrically insulated,and allow a high degree of freedom of peripheral circuits compared tothe 3-terminal Rosen-type piezoelectric transformer shown in FIG. 1.

Third Embodiment!

FIG. 5 shows a perspective view of the third embodiment of thepiezoelectric ceramic transformer of the present invention. The entirestrip-structure piezoelectric ceramic transformer is broadly dividedinto two parts in the direction of the width: driver 11 and generator12. As shown by arrow 17 in the figure, driver 11 is constructed withexternal electrodes 131, 132 arranged on the upper and lower mainsurfaces of a piezoelectric ceramic 10 polarized in the direction of itsthickness, with external electrical terminals 151, 152 leading out fromthe central portion in the direction of length. Generator 12 isconstructed with opposing interdigital electrodes 141 and 142 arrangedon one of the main surfaces of piezoelectric ceramic 10 polarized inalternating directions indicated by arrows 18 between the electrodefingers of the interdigital electrodes, with external electricalterminals 161, 162 being led out from the central portions, in thedirection of length, of interdigital electrodes 141, 142.

When an alternating voltage is applied between external electricalterminals 151-152 of the driver of the construction shown in FIG. 5,longitudinal vibration is generated in the longitudinal direction of theentire piezoelectric ceramic transformer due to piezoelectric transverseeffect 31 mode through electromechanical coupling factor k₃₁.Accordingly, in generator 12, a voltage is generated betweeninterdigital electrodes 141-142 due to the piezoelectric longitudinaleffect 33 mode through electromechanical coupling factor k33, and thevoltage is led out from external electrical terminals 161-162. At thistime, if the frequency of the applied voltage is equal to the resonancefrequency of longitudinal vibration of the piezoelectric ceramictransformer, a considerably high output voltage can be obtained.

Operation is possible if interdigital electrodes 141, 142 as explainedabove are arranged on the main surface of only one side, but needless tosay, an arrangement having interdigital electrodes in matched positionson both upper and lower main surfaces provides higher generationefficiency.

The lumped constant approximate equivalent circuit in the vicinity ofthe resonance frequency of this piezoelectric ceramic transformer, asfor the first and second embodiments of the piezoelectric transformer,is shown in FIG. 4. Accordingly, the characteristics and advantages ofthis embodiment are as for the first and second embodiments.

Fourth Embodiment!

The fourth embodiment, a piezoelectric ceramic transformer ofrectangular plate structure, is shown in FIG. 6. This piezoelectricceramic transformer is provided with a rectangular piezoelectric ceramicplate 10, the entire piezoelectric ceramic transformer being dividedbetween driver 11 and generator 12. Driver 11 is arranged with opposinginterdigital electrodes 111, 112 arranged on the main surface of driver11. Each interdigital electrode is constructed of numerous electrodefingers extending along the width of the piezoelectric ceramictransformer and one connecting member electrically linking theseelectrode fingers. Each electrode finger of one interdigital electrodeprojects between a pair of electrode fingers of the other interdigitalelectrode, the electrode fingers opposing each other in the longitudinaldirection of the piezoelectric ceramic plate 10. In contrast with a casein which the entire device is uniformly polarized such as a surface-wavedevice, portions of the plate between opposing electrode fingers arealternately polarized in a longitudinal direction as shown by the arrowsin the figure. As for the connecting member, external electricalterminals 113, 114 are led out from the center of the length of driver11.

Generator 12, as with driver 11, is arranged with opposing interdigitalelectrodes 121, 122 and is alternately polarized in the longitudinaldirection between each pair of electrode fingers as shown by the arrowsin the figure. External electrical terminals 123, 124 are led out at thecenter of the length of generator 12.

Next will be explained the operation of the present embodiment. When analternating voltage is applied between external electrical terminals113-114 of driver 11 of the construction shown in FIG. 6, strain in thelongitudinal direction is generated between each pair of electrodefingers due to the piezoelectric longitudinal effect 33 mode throughelectromechanical coupling factor k₃₃. At this time, because thedirection of polarization and the direction of the applied electricfield are both alternating, the portions of the plate between electrodefingers of driver 11 all repeat expansion and contraction at the samephase. As a result, a longitudinal vibration is generated in thelongitudinal direction throughout the piezoelectric ceramic transformer.This longitudinal vibration is propagated in generator 12; and in thegenerator as well, a voltage is generated due to piezoelectriclongitudinal effect 33 mode through electromechanical coupling factork₃₃ between each pair of electrode fingers.

At this time, if driven at the same frequency as the resonance frequencyof the longitudinal vibration in the longitudinal direction, a highvoltage can be obtained in generator 12, and if a second order mode(1-wavelength mode) is employed, vibration nodes will occur at points1/4 of the overall length of the piezoelectric ceramic transformer fromboth end surfaces. According to the construction of the presentinvention, extremely high reliability can be achieved because all of theexternal electrical terminals are led out from these nodes.

Also, it will be easily understood that m drivers 11 and n generators 12can make a piezoelectric ceramic transformer in the same way asexplained in the second embodiment, which is driven at the resonancefrequency of longitudinal vibration of (m+n)th order.

As in the piezoelectric transformers of the first to third embodiments,the lumped constant approximate equivalent circuit of the vicinity ofthe resonance frequency for this piezoelectric ceramic transformer isshown in FIG. 4. Accordingly, the characteristics and advantages of thisembodiment are as for the previously described embodiments.

Fifth Embodiment!

The fifth embodiment of the piezoelectric ceramic transformer accordingto the present invention is shown in FIG. 7. The electrode of driver 11is the same as for the fourth, but in generator 12, instead of aninterdigital electrode, a strip electrode 125, 1 mm wide, is arranged atthe center of the output section with one external electrical terminal126 leading out and the other external electrical terminal 127connecting to external electrical terminal 114 of driver 11, and inaddition, the electrode fingers of driver 11 are 0.3 mm wide. Regardingpolarization processing after external terminal connection, voltage isapplied to electrodes 111-112 in driver 11, but in generator 12,polarization is between electrode 112 of driver 11 and electrode 125 ofgenerator 12. As a result, only half of the region of generator 12 isactivated as a piezoelectric body; however, this arrangement issufficient for a low-power, three-terminal construction.

Next, an explanation will be given of the manufacturing method of theinterdigital electrodes used in each of the above-explained embodiments.

As shown in FIG. 8, a case is considered in which opposing interdigitalelectrodes 51, 52 are formed on the surface of rectangular piezoelectricceramic 50 which is still inactive after baking. Interdigital electrode51 is formed from electrode fingers 53 and connecting electrode 56 whichlinks electrode fingers; and interdigital electrode 52 is formed fromelectrode fingers 55 and connecting electrode 56 which links electrodefingers. These interdigital electrodes may be formed by printing with aconductive paste and baking, by a sputtering method, by a vacuumevaporation method, or by any method that does not chemically orphysically damage the piezoelectric ceramic.

After forming an interdigital electrode, a polarization process iscarried out to activate the material as a piezoelectric substance(arrows in the figure indicate the direction of polarization). Tofacilitate polarization at this time, the electrode is placed in ahigh-temperature state at a level below the Curie point, and anextremely high direct-current voltage V_(pole) is applied betweeninterdigital electrodes 51-52, a field intensity of a level of 3-4 kV/mmbeing required for a typical PZT ceramic. Because this field intensityapproaches the field intensity at which dielectric breakdown occursbetween electrodes, the distance between two opposing electrodes must beat least a set amount. In other words, the distance d₂₂ between anelectrode finger and a connecting electrode must be at least thedistance d₂₁ between electrode fingers 53-55. As a result, there is theproblem that the distance l₂ along which electrode fingers 53, 55 opposeeach other decreases, thereby decreasing the longitudinal polarizedregion that can be effectively used as piezoelectric substance. Inaddition, piezoelectric ceramics when polarized are characterized byexpansion in the direction of the electric field and contraction in adirection perpendicular to the electric field compared with thenon-polarized state. In a piezoelectric ceramic transformer manufacturedaccording to this method, the region of expansion in the direction oflength mixes with the region perpendicular to this of expansion in thedirection of width, giving rise to the problem that stress, andconsequently a tendency for breakdown, is created in the vicinity of theborders of these regions.

In the present embodiments described above, as shown in FIG. 9,strip-shaped electrodes 11 are arranged on the surface of rectangularpiezoelectric ceramic 10 which is inactive after baking. The length ofthese electrodes is set at a length equal to or greater than the overallwidth of the interdigital electrode to be formed. Electrically andalternately connected lead lines 12, 13 are led out from the stripelectrodes 11 and the polarization voltage V_(pole) is applied (arrowsin the figure indicate the direction of polarization). After completionof the polarization process, the lead lines 12, 13 and the stripelectrodes 11 are removed.

Subsequently, as shown in FIG. 9(b), opposing interdigital electrodes14, 15 are formed at a temperature equal to or less than the Curiepoint. Interdigital electrode 14 is made up of electrode fingers 16 andconnecting electrode 17 which links the electrode fingers; andinterdigital electrode 15 is made up of electrode fingers 18 andconnecting electrode 19 which links the electrode fingers. At this time,electrode fingers 16, 18 of interdigital electrodes 14, 15 are alignedwith the positions of original strip electrodes 11.

When interdigital electrodes 14, 15 are formed by this process, a highvoltage for polarization is not applied between the opposinginterdigital electrodes, and insulation need only withstand therelatively low voltage used during operation as a piezoelectric ceramictransformer. Consequently, the distance d₁₂ between electrode fingersand an opposing connecting electrode may be less than the distance d₁₁between opposing electrode fingers, and to this extent, the distance l₁of opposition between electrode fingers 16, 18 can be made greater,thereby allowing a larger portion of the piezoelectric ceramic to bepolarized in the direction of the longitudinal arrows and to functioneffectively.

Further, as is clear from FIGS. 9(a) and 9(b), because the entirepiezoelectric ceramic is polarized in only the longitudinal direction ofthe piezoelectric ceramic transformer, there are no adjacent regions ofdiffering polarization direction, and the occurrence of mechanicalfailure can therefore be suppressed.

Next will be explained the manufacturing method of the above-describedfourth embodiment shown in FIG. 6 as an example.

A piezoelectric ceramic PZT (PbZrO₃ -PbTiO₃) was used as thepiezoelectric ceramic material.

First, a baked piezoelectric ceramic block is cut using a diamondcutter, and a piezoelectric ceramic plate 10 measuring 30 mm long, 8 mmwide, and 1.0 mm thick is prepared by grinding using #3000 SiC grindingpowder. Aluminum strip electrodes (not shown in the figure) are formedon the piezoelectric ceramic plate 10 by a vacuum evaporation methodusing a metal mask. Electrical terminals (not shown) are alternately ledout from the strip electrodes using for connection an Ag paste hardenedat 150° C.

Next, the driver and generator are both treated by a polarizationprocess in which a voltage of 4 kV/mm is applied in insulating oil at100° C. After the polarization process, the Ag paste is removed by anorganic solvent, and after removing the aluminum electrodes with a KOHetching liquid, interdigital electrodes of Au/Ti composition are formedby a vacuum evaporation method using a metal mask.

In FIG. 6, the interdigital electrodes of driver 11 are indicated as 111and 112, and the interdigital electrodes of generator 12 are indicatedas 121 and 122. To keep the temperature of the piezoelectric ceramicbelow the Curie point during vacuum evaporation, plate heating is notused, and in addition, external electrical terminals 113, 114, 123, 124are connected by soldering.

In the above-described manufacturing method, any removable conductivematerial can be used instead of Al for the strip electrodes, and anymethod that does not damage the piezoelectric ceramic can be used as theforming method, including a sputtering method, baking method, or platingmethod. Furthermore, reactive ion etching (RIE) or physical grinding maybe used instead of etching liquid. Formation of the interdigitalelectrodes following polarization may also be carried out by methodsother than described above, but formation must occur at a temperaturebelow the Curie point in order that polarization of the piezoelectricceramic may not be affected.

A maximum output of 1.5 W can be obtained from a piezoelectric ceramictransformer produced in this way, and this represents a largeimprovement over the maximum output of 1.1 W of the piezoelectricceramic transformer of the same outside dimensions in which thepreviously described polarization process is performed after formationof the interdigital electrodes.

In addition, of 100 piezoelectric ceramic transformers manufacturedaccording to this method, not one sample experienced mechanical failureduring polarization or during any of the following production processes.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

What is claimed is:
 1. A method for manufacturing a piezoelectricceramic transformer having an interdigital electrode assembly providedon a surface of the piezoelectric ceramic comprising the stepsof:providing an inactive piezoelectric substrate having its largestdimension in a longitudinal direction; disposing on a surface of saidpiezoelectric substrate strips of conductive material, saidpiezoelectric substrate strips being provided by a coating operation;energizing said strips with high polarizing voltage so as to polarizesaid piezoelectric substrate in only the longitudinal direction of saidpiezoelectric substrate; removing said strips from said surface; andforming conductive electrodes on said surface of said piezoelectricsubstrate, said conductive electrodes being aligned with the positionsoriginally occupied by said strips.
 2. The method of claim 1, whereinsaid strips traverse an entire width of said substrate, and wherein thelength of said strips is at least equal to the overall width of theinterdigital electrodes to be formed.
 3. A method for manufacturing apiezoelectric ceramic transformer having an interdigital electrodeassembly provided on a surface of a piezoelectric ceramic plate; a firstpart of the interdigital electrode assembly being made up of a pluralityof electrode fingers spaced at intervals and parallel to the directionof width of the piezoelectric ceramic plate and a connecting electrodelinking these electrode fingers; said first part and a second part ofsimilar construction to said first part being arranged such thatelectrode fingers of both said first and second parts alternatelyproject so as to oppose each other across the width of the piezoelectricceramic plate; comprising the steps of:providing an inactivepiezoelectric ceramic substrate having its largest dimension in alongitudinal direction; disposing on a surface of said piezoelectricsubstrate strips of conductive material, said piezoelectric substratestrips being formed by a coating operation, each traversing the width ofsaid surface; energizing said strips with high polarizing voltage so asto polarize said piezoelectric substrate in only the longitudinaldirection of said piezoelectric substrate; removing said strips fromsaid surface; forming said first part of said interdigital electrodeassembly by forming said plurality of electrode fingers on said surfaceof said piezoelectric substrate on alternating locations previouslyoccupied by respective ones of said strips; forming said second part ofsaid interdigital electrode assembly by forming a second set ofplurality of electrode fingers on said surface of said piezoelectricsubstrate on remaining alternating locations previously occupied byother ones of said strips; and forming connecting electrodes, eachlinking said electrode fingers and said second set of plurality ofelectrode fingers, respectively.
 4. The method for manufacturing aninterdigital electrode assembly according to claim 3, wherein thedistance between said electrode fingers and an opposing connectingelectrode is less than the inter distance between opposing electrodefingers.